Present day television receivers frequently encounter the problem of properly handling signals of a relatively large magnitude to provide relatively distortion-free and cross-modulation free signals. Thus, TV tuner apparatus must include some provision for dealing with relatively large received signals.
One known form of apparatus for handling relatively large TV signals is set forth in U.S. Pat. No. 3,813,602 issued May 28, 1974. The circuitry includes an attenuator circuit employing a first PIN diode shunting the signal source to a to a potential reference level and a second PIN diode AC coupled in series with an amplifier stage. As the magnitude of a received signal increases, an AGC potential is applied to the second PIN diode so that current flow therethrough decreases and the impedance value increases. Moreover, a decrease in current flow through the second PIN diode is accompanied by an increase in current flow through the first or shunting PIN diode. Thus, the attenuator circuit serves to provide a desirable level of output signal despite application of a signal having an undesired large magnitude.
Although the above-described apparatus does provide advantages and enhanced results over prior known apparatus, it has been found that there are a number of features which leave something to be desired. For example, AC coupling of the amplifier stage to the second PIN diode inhibits any alteration in bias, and consequently gain, of the amplifier stage as current flow through the PIN diode is decreased due to the AGC action. Thus, only a linear change in signal due to the linear impedance characteristics of the PIN diode are experienced whereupon a limited linear attenuation in the signal to noise ratio with respect to gain is achieved.